Patients with psoriasis and psoriatic arthritis are candidates to receive anti-TNF alpha therapies which require prior screening for latent tuberculosis (LTB). Currently, screening for LTB is based on tuberculin skin test (TST) , chest X rays, and a questionaire on predisposing factors for TB. The main drawbacks of TST are the lack of specificity due to cross reactivity with Bacille Calmette- Guerin (BCG) and other nontuberculosis mycobacteria and the risk of anergy in immunosuppressed patients. Furthermore, it has been suggested that the skin of psoriatic patients may be more sensitive resulting in increased TST which does not obligatory reflect the status of LTB.